Derepression of genes on human inactive X chromosomes will be studied by treatment of mouse cells that contain an inactive X and human cells that are heterozygous for alleles of the HPRT and G6PD loci with agents that perturb DNA methylation as well as chemical and physical mutagens. Autoradiography will be used to determine if the timing of replication of some DNA regions is advanced in cells in which derepression of X-chromosomal genes has occurred and in order to estimate the length of such regions. Probes made with DNA clones containing X-chromosomal inserts will be used to determine if specific segments of the X are on different fractions of diverse restriction enzyme digests of DNA from active and inactive X's. Chemicals and UV will be used to transform human diploid cells toward malignancy. The rates of change and mutagen specificity with regard to morphological transformation, anchorage independent growth and tumorigenicity in the nude mouse will be determined. Permanent lines of diploid, untransformed cells will be created by means of transfer of specific chromosomes from permanent lines into senescible human cells. The lines will be used to study stepwise transformation toward malignancy and as recipients of cancer cell DNA.